Combustion of fossil fuel in a gas turbine engine produces a number of objectionable combustion byproducts including nitrous oxides (NOx). Regulatory authorities often impose stringent limits on turbine engine NOx emissions, especially when those emissions are produced by stationary, ground based engines used as industrial powerplants. Accordingly, turbine engine manufacturers continually seek improved ways to inhibit NOx formation.
One of the principal NOx suppression strategies is to burn a stoichiometrically lean, thoroughly blended fuel-air mixture. Typically, fuel and air are aggressively blended together in an internal mixing chamber of a premixing fuel injector before being introduced into the engine combustion chamber and burned. The lean, thoroughly blended fuel-air mixture results in a uniformly low combustion flame temperature--a prerequisite for NOx suppression.
Although fuel injectors that premix fuel and air are effective at producing the requisite, intimately blended fuel-air mixture, they suffer from certain shortcomings. For example, the presence of the fuel-air mixture inside the injector can encourage the combustion flame to migrate into the mixing chamber where the flame can cause considerable damage. Accordingly, premixing fuel injectors have a number of physical features designed to resist flame ingestion and to quickly disgorge any flame that overcomes the ingestion resistance. Despite these features, a flame can occasionally become anchored inside the mixing chamber. Therefore a premixing injector may also have one or more temperature sensors to detect the presence of flame so that appropriate corrective action can be taken. In one existing arrangement, the temperature sensor is a thermocouple welded to the interior of the injector with its sensing junction positioned near the mixing chamber.
Although the welded thermocouple is effective for monitoring internal temperature, it is not easily replaceable. If the thermocouple malfunctions, maintenance technicians must first remove the affected fuel injector from the engine. The fuel injector is then disassembled, the weld joints are broken to release the inoperative thermocouple, and a replacement thermocouple is welded into position. Finally, the injector is reassembled and reinstalled in the engine. Clearly, this procedure is unacceptably time consuming and labor intensive. Moreover, industrial operations are disrupted and operating revenue is sacrificed while the engine is out of service. An engine operator may keep one or more spare injectors on hand to minimize the length of service disruptions. However, this option is unappealing because of the expense of acquiring and stockpiling spare injectors.
What is needed is a premixing fuel injector having conveniently replaceable means for internal temperature monitoring.